Liquid crystals are used, in particular, as dielectrics in display devices since the optical properties of such substances can be affected by an applied voltage. Electrooptical devices based on liquid crystals are extremely well known to those skilled in the art and may be based on various effects Devices of this type are, for example, cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SBE (super-birefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability toward electrical fields and electromagnetic radiation, especially in the visible and ultraviolet spectral range. Furthermore, the liquid-crystal materials should have low viscosity and give short addressing times, low threshold voltages and high contrast in the cells. Furthermore, they should have a suitable mesophase, for example, for the abovementioned cells, a nematic or cholesteric mesophase, at customary operating temperatures, i.e. generally in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For display devices addressed with low multiplex ratios (common use displays), which are the preferred subject-matter of the present invention, nematic mixtures having clearing points above 55.degree. (preferably above 65.degree.), birefringence in the range from 0.04 to 0.33 (preferably from 0.07 to 0.23) and dielectric anisotropy in the range from 1.5 to 39 (preferably from 4 to 19) are desired.
A known mixture from the prior art is E7 (BDH, Poole, Great Britain), which contains cyanobiphenyls and has the following parameters EQU T.sub.N,I =60.5.degree. EQU .DELTA.n=0.225 EQU .eta..sub.20 =39 cSt EQU V.sub.10 (TN)=1.5 volts
For many applications, however, mixtures of this type have undesired high viscosity and/or excessively low dielectric anisotropy, which results in undesirably long switching times for excessive threshold voltages.